Nonvolatile memories such as flash memories can be widely used as storage media in various systems. Some nonvolatile memories have cells capable of storing one bit each, while other enables storing two or more bits in each memory cell. Such memory cells are also referred to as multi-level cells (MLC) memories.
In a four-level flash memory, every cell may store two bits. In a program or read operation a whole page may be usually processed. A page may be constructed from a set of cells, and may include a single bit from each cell. Therefore, in a four-level memory there can be two pages defined—a Least Significant Bit (LSB) page referring to all LSB bits of the same string (or set of cells), and a Most Significant Bit (MSB) page.
In an eight-level MLC configuration, each memory cell may store three bits, which are referred to as a MSB, a Central Significant Bit (CSB) and a LSB. In a common reading scheme, the MSB can be read using a single threshold, the CSB can be read using a set of two thresholds, and the LSB can be read using a different set that may include four thresholds.
The flash memory page may be programmed in a gradual manner—programming the page using a single MSB threshold, programming the page using three CSB program thresholds, and programming the LSB page using seven program thresholds. The flash memory page can be programmed (at the beginning of the life cycle of the flash memory) so that the threshold voltages of the different flash memory cells may be within non-overlapping lobes. Over time the lobes may be shifted and deformed, thus, causing read errors.